The objective of this work is to relate the interactions between myosin, actin, tropomyosin troponin, calcium, and MgATP to the contraction and relaxation of muscle; and to the enzymatic steps in the hydrolysis of ATP which supplies the energy for mechanical work. Optical methods are being devised and used for studying the kinetics of the various protein-protein interactions and these are then related to kinetic studies of the enzymatic mechanism for ATP hydrolysis. Subfragments of myosin are employed in this work to further pinpoint the function of different parts of this key protein. Chemical modification, kinetics of superprecipitation, the kinectics of O 18 exchange, and studies on temperature effects are some of the other methods and parameters used in these studies.